1. Field of the Invention
This invention relates to methods and subsystems for determining a sequence in which microstructures are to be processed at a laser-processing site.
2. Background Art
Memory repair is a process used in the manufacture of memory integrated circuits (DRAM or SRAM) to improve the manufacturing yield. Memory chips are manufactured with extra rows and columns of memory cells. During testing of the memory chips (while still in the wafer form), any defects found are noted in a database. Wafers that have defective die can be repaired by severing links with a pulsed laser. Systems generally utilize wafer-handling equipment that transports semiconductor wafers to the laser process machine, and obtain the information in the form of an associated database specifying where the links should be cut and perform the requisite link ablation for each wafer.
Successive generations of DRAM exploit finer device geometry in order to pack more memory into smaller die. This manufacture of smaller devices affects the geometry of the links allocated for laser redundancy. As the devices get smaller, the links get smaller and the pitch (link-to-link spacing) shrinks as well. Smaller link geometry requires a smaller spot size from the laser in order to successfully remove selected links without affecting adjacent links, preferably with little if any compromise in throughput.
Links are arranged in small groups on a common pitch. Typical groups might consist of 20-50 links that are about 0.8 xcexcM wide, 6 xcexcM long and spaced on a 3 xcexcM pitch. The groups can be arranged with the pitch vector parallel to either the x or y axis. Frequently, multiple groups will be found to be co-linear with as little as 2 or 3 xe2x80x9cpitchesxe2x80x9d between the end of one group and the start of the next. At other times, the co-linear groups may be spaced many hundreds of microns apart. A typical DRAM die might have a few hundred groups of links distributed across the extend of the die.
The above-noted utility patent application discloses a high-performance x-y stage that is used to position a semiconductor wafer underneath a stationary laser beam. The x-y stage is required to make many high-speed motions in the course of processing a region of the wafer.
Typical x-y stages are limited by position, velocity, acceleration and voltage constraints. The position constraints are the limits of travel of the stage. The velocity constraints may be due to limits on the positioning sensing devices used in the stage and/or limits imposed by the type of bearings used. Acceleration constraints are usually due to limits on the current available to drive the stage due to motor, amplifier or power supply considerations. Voltage constraints are encountered less frequently. Movements that require excess voltage must be avoided in order to avoid amplifier saturation. Voltage constraints are typically encountered during brief high-acceleration moves.
The stage is able to execute a motion using a relatively large value of acceleration (as high as 3 G""s) and to stop rapidly at the end of the motion. The stage also must operation with extremely high precision (nanometer level of precision). The combination of very high performance and extreme accuracy requirements places additional constraints of the stage. The stage is sensitive to the level of power dissipated in the motor windings. If the power dissipated causes the winding temperature to rise more than 2xc2x0 C., then the precision of the stage will be compromised. Also, if the spectral content of the forces applied to the stage contains an excessive level of energy at frequencies above the first mechanical mode of the system, then resonances in the mechanical elements of the stage will be excited. These resonances will either compromise accuracy or require extra settling time at the end of a motion before the requisite level of precision will be achieved.
An object of the present invention is to provide an improved method and subsystem for determining a sequence in which microstructures are to be processed at a laser-processing site.
In carrying out the above object and other objects of the present invention, a method for determining a sequence in which microstructures are to be processed at a laser-processing site is provided. The method includes receiving reference data which represent locations of microstructures to be processed at the site. The method also includes coalescing adjacent groups of microstructures into clusters of microstructures including edge clusters which contain microstructures located near travel limits of a motor-driven stage which moves the microstructures relative to a laser beam at the site. The method also includes dividing a cluster fragment from each edge cluster wherein the cluster fragments contain the microstructures located near the travel limits. The method further includes sorting the clusters and cluster fragments to obtain data which represent a substantially optimum sequence in which the microstructures are to be processed to increase throughput at the site.
The step of sorting may be based on energy expended in at least one coil of at least one motor in response to motor commands.
Each of the cluster and cluster fragments may have a plurality of possible processing directions and wherein the step of sorting may include the step of determining a substantially optimum direction in which to process the micro structures.
The step of sorting may include the steps of selecting a substantially optimum cluster or cluster fragment to be initially processed at the site, then determining a plurality of possible sequences for processing the remaining clusters and cluster fragments and selecting a substantially optimum sequence from the plurality of possible sequences.
The microstructures may be located on dice of a wafer.
Further in carrying out the above object and other objects of the present invention, a subsystem for determining a sequence in which microstructures are to be processed at a laser-processing site is provided. The subsystem includes means for receiving reference data which represent locations of microstructures to be processed at the site. The subsystem also includes means for coalescing adjacent groups of microstructures into clusters of microstructures including edge clusters which contain microstructures located near travel limits of a motor-driven stage which moves the microstructures relative to a laser beam at the site. The subsystem further includes means for dividing a cluster fragment from each edge cluster wherein the cluster fragments contain the microstructures located near the travel limits. Further, the subsystem includes means for sorting the clusters and cluster fragments to obtain data which represent a substantially optimum sequence in which the microstructures are to be processed to increase throughput at the site.
The means for sorting may sort based on energy expended in at least one coil of at least one motor in response to motor commands.
Each of the clusters and cluster fragments may have a plurality of possible processing directions and wherein the means for sorting includes means for determining a substantially optimum direction in which to process the microstructures.
The means for sorting may include means for selecting a substantially optimum cluster or cluster fragment to be initially processed at the site, for determining a plurality of possible sequences for processing the remaining clusters and cluster fragments and for selecting a substantially optimum sequence from the plurality of possible sequences.
The microstructures may be conductive lines of the dice and wherein the conductive lines may be metal lines.
The dice may be semiconductor memory devices and wherein the conductive lines are to be ablated at the site to repair defective memory cells of the devices.
The microstructures may be parts of a semiconductor device and wherein the semiconductor device may be a microelectromechanical device.
The semiconductor device may also be a silicon semiconductor device.
The semiconductor device may further be a semiconductor memory.
The microstructures may be parts of a microelectronic device.
The microstructures in each group may have a substantially common pitch.
The stage is may be an x-y stage and wherein the means for sorting may sort based on energy expended in a plurality of coils of a plurality of motors in response to motor commands.
The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.